Recently, a short arc discharge lamp has rapidly been spreading as a light source of a liquid crystal projector. Such a kind of lamp is used as a light source apparatus constructed by combining with a reflecting mirror having a reflective surface formed on the internal surface of the reflecting mirror as a paraboloid of revolution or an ellipsoid of revolution for collecting light to a liquid crystal panel. Moreover, such a light source apparatus is required to be more miniaturized and to have a higher efficiency as a light source apparatus for a mobile projector. Accordingly, various proposals for the downsizing of the light source apparatus and for utilizing the reflective surface of a reflecting mirror efficiently have been made.
U.S. Pat. No. 5,506,464 shows the lamp unit which consists of the reflector and discharge lamp for such a light source apparatus.
FIG. 5 shows the reflector 31 used for such a lamp unit. The reflector 31 has a hollow neck portion 33 for holding one of seal parts of a lamp formed in the bottom of the reflector body 32 which has a rotation ellipse surface that will be a reflective surface 31a. The inside surface 33a of the hollow neck portion 33 spreads toward a back opening.
As shown in FIG. 6 (a), this reflector 31 is manufactured by using the outer mould 34 and the inner mould 35. First, the glass gob G is put in outer mould 34. Second, the glass is pushed and extended by the inner mould 35.
As shown in FIG. 6(b), the glass G is filled up in the cavity 36 formed among outer mould 34 and inner mould 35 and the reflector body 32 is made. At this time, a partition 37 which separates the reflecting surface 31a and hollow neck portion 33 is formed between the top 35a of the inner mould 35 and the core 34a of the outer mould 34. Therefore, it is necessary to open the partition 37 and to form the lamp insertion hole 38.
In order to form the lamp insertion hole 38, as shown in FIG. 6 (c), the rotatable cylindrical grindstone 39 grinds the partition 37. Then, a reflective membrane is formed on the inner surface of the reflector body 32 by vapor deposition. As shown in FIG. 6 (d), such a mirror 31 has a narrowest portion 40 at the reflective surface side of the hollow neck portion 33. The narrowest portion 40 functions as a dam which prevents the outflow of the adhesives 52 which fix a lamp (refer to FIG. 5).
However, since the insertion hole 38 is opened by the grindstone, as shown in FIG. 6 (d), there are countless fine scratches on the inner surface of the hole 38. Therefore, when the reflector 31 was used for a lamp unit, a crack occurs at the reflective surface side of the hole 38 by high temperature of lamp, and finally, the reflector 31 will be broken. Since especially the opening edge 38a of the insertion hole 38 tended to be influenced of the heat of a lamp, the reflector 31 breaks very easily.
For removing scratches, from the past, the process of polishing by flame of burner 41 for finish of the inner surface of the insertion hole 38 had to be carried out further as shown by FIG. 7, so that the narrowest portion 40 with countless scratches becomes a smooth surface without any scratches. Thus manufacturing cost was increasing.
If a partition 37 was not formed, since it is not necessary to open the hole 38 by grindstone, there is not such a problem. Inventors made trial moulds which were designed so that tip 35a of the inner mould 35 and the top 34b of the core 34a of outer mould 34 are joined each other. And they tried to manufacture the reflector using the moulds. However, glass solidifies under cooling rapidly and a partition is formed, if the gap of tip 35a and top 34a is set to 1 mm or less while extending glass gob by the inner mould 35. Thereby, since tip 35a does not join to the top 34a, a reflector 31 can not be fabricated according to the design.
Therefore, the object of the present invention is as follows; no scratches by grindstone of narrowest portion on inner surface of insertion hole without the process of polishing by flame, when an insertion hole was opened in the partition of the reflector body by grindstone